1. Field of the Invention
The present invention relates to an improved device for feeding an ink ribbon used in a printing machine, and particularly to a mechanism for changing the running direction of the ink ribbon by detecting the end portions of the ink ribbon.
2. Description of the Prior Art
There have so far been proposed several types of mechanisms for changing the running directions of ink ribbons in printers. Among them, representative examples will include a method by which a feeding pawl for effecting reciprocating operation is alternately engaged with the right and left winding wheels on which an ink ribbon is wound after an end portion of the ink ribbon has been detected thereby to change the running direction of the ink ribbon, and a method by which an intermediate gear meshing with a main drive gear which rotates in one direction is alternately engaged with the right and left winding wheels on which the ink ribbon is wound after an end portion of the ink ribbon has been detected in order to change the running direction of the ink ribbon. In order to change the running direction of the ink ribbon by means of either the feeding pawl or the intermediate gear, the abovesaid two methods employed a spring as shown, for example, in U.S. Pat. No. 3,786,906.
With the so-called deviation-producing mechanisms using the spring mentioned above, however, the stable positions are present only at both extreme ends with no stable point at the center. Besides, due to the nature of the changing mechanism, the central portion contains a so-called non-sensitive zone in which the mechanism responds neither to the driving force from the right side nor to the driving force from the left side. Therefore, the conventional mechanisms for changing the running direction of the ink ribbon presented a probability that the changing member (feeding pawl, or intermediate gear) went into halt in the non-sensitive zone. Consequently, according to the conventional mechanisms, the running ink ribbon often came into halt, causing the concentration of the printed characters to be decreased and giving a great damage to the ink ribbon.
Anoter method employed the winding force of the ink ribbon to actuate the changing member. According to this method, however, excess of tensile force was exerted on the ink ribbon when the running direction was to be changed, whereby the printing energy was absorbed in case of the flying-type printing hammers giving rise to the development of non-uniform printing force, deteriorating the quality of printed characters, giving damage to the ink ribbon, and reducing the durability of the ink ribbon.